PR #19333 - BasedHardware/omi

Closes #19332

Hardens backend/database/webhook_health.py against serialization failures, unhandled exceptions, and in-memory cache poisoning:

DLQ Non-JSON Payload Serialization: Added default=str to json.dumps in enqueue_dev_webhook_dlq so payloads containing datetime, UUID, or other non-primitive types serialize gracefully instead of failing silently and dropping the event from the DLQ.
Subscript Crash Guard: Guarded (webhook_url or '')[:500] to prevent TypeError: 'NoneType' object is not subscriptable when webhook_url is None.
Firestore Auto-Disable Resilience: Handled google.api_core.exceptions.NotFound gracefully in disable_app_in_firestore. Cache state is now updated only on successful Firestore update, preventing cache poisoning if Firestore write fails.
Resilient Cache Eviction: Protected _evict_oldest against empty dictionary input (StopIteration risk) and added safe type casting for mixed cache entries.
Input Validation: Normalized and stripped string identifiers across record_app_webhook_failure, record_app_webhook_success, clear_app_webhook_health, is_app_webhook_disabled, get_app_webhook_health, record_dev_webhook_failure, and record_dev_webhook_success.

Authored hermetic unit tests in backend/tests/unit/test_database_webhook_health_resilience.py (10 passing tests).
Verified existing suite backend/tests/unit/test_webhook_health_cache_cap.py (2 passing tests).
Formatted with Black 26.5.1 (--line-length 120 --skip-string-normalization) with 0 formatting deviations.

#19333 fix(backend): validate inputs and handle serialization in database webhook health